1. Field of the Invention
The present invention relates to a discharge lamp for an automobile headlight and to the automobile headlight.
2. Description of the Prior Art
Conventionally, a halogen lamp generally has been used in an automobile headlight. Recently, a headlight employing a high pressure discharge lamp (referred to as a HID headlight) has been increasingly widespread as a headlight that can achieve energy saving and improved brightness. The HID headlight can illuminate forward more brightly with a small amount of power than a headlight employing a halogen lamp. On the other hand, the brightness of the HID headlight dazzles and causes uncomfortable glare to the drivers in the automobiles running in the opposite direction. This may lead to a car accident. Therefore, a headlight that illuminates forward brightly and hardly causes undesired glare is desired.
The uncomfortable glare can be alleviated by reducing the intensity of light of the headlight toward the opposing automobile, or lowering the correlated color temperature of the light source. The former approach is not desirable because the illumination in the forward direction is reduced. One example of the latter approach may be to use a high pressure discharge lamp with a low correlated color temperature such as a high pressure sodium lamp. However, the high pressure sodium lamp emits yellowish light and has a low color rendering property, thereby interfering with the recognition of the safety color of road marking or traffic signs, although a risk of car accidents may be smaller.
For a conventional metal halide lamp for an automobile headlight, a luminous tube lamp having an outer tube including a multi-layered interference film formed on the surface thereof has a color temperature of more than 4000K has been proposed to obtain, for example, a low color temperature of 2000 to 4000K that is close to the color of an incandescent lamp (Japanese Laid-Open Patent Publication No. 5-325895). Furthermore, for a conventional metal halide lamp, a luminous tube lamp having a correlated color temperature of 2800 to 3700K that has a color close to the color of blackbody radiation has been proposed (Japanese Laid-Open Patent Publication No. 7-130331).
However, the above-described discharge lamps have a problem in that they fail to provide a color discrimination (identification) property as good as or similar to that of a fluorescent lamp used for intramural illumination.
Furthermore, for a conventional metal halide lamp, a luminous tube enclosing a metal halide and argon gas therein has been proposed (Japanese Laid-Open Patent Publication Nos. 7-320688, 7-130331, 62-131459, 61-64060, and 5-205697).
However, these conventional metal halide lamp have poor luminous flux rising characteristics, so that they fail to illuminate forward immediately after they are switched on. Therefore, they are not suitable for a discharge lamp for an automobile.
Therefore, with the foregoing in mind, it is an object of the present invention to provide a discharge lamp for headlights that hardly creates uncomfortable glare and provides easier discrimination of various colors including the safety color, and a headlight employing such a discharge lamp.
In order to achieve the object, a first discharge lamp of the present invention includes a luminous tube in which xenon gas is sealed, and has a light color that lies in a chromatic region common to the following regions: a region bounded by an ellipse with a color point (u, v)=(0.224, 0.331) as the center thereof, a major axis of 0.080, a minor axis of 0.024, and an angle from the u axis of 35 degrees in the CIE 1960 UCS diagram; a region bounded by an ellipse with a color point (u, v)=(0.220, 0.332) as the center thereof, a major axis of 0.060, a minor axis of 0.022, and an angle from the u axis of 15 degrees in the CIE 1960 UCS diagram; and a region bounded by an ellipse with a color point (u, v)=(0.235, 0.335) as the center thereof, a major axis of 0.060, a minor axis of 0.030, and an angle from the u axis of 30 degrees in the CIE 1960 UCS diagram. Herein, the CIE 1960 UCS diagram is a chromaticity diagram standardized in 1960 by the CIE (Commission Internationale de l""Eclarage in French), as defined in Japanese Industrial Standards (JIS) Z8105. The CIE 1960 UCS diagram is designed to allow measurement of color differences, and calibrated such that with respect to all points in the diagram, a difference in colors perceived by observers when viewing the colors having the same brightness is in proportion to a geometric distance in the diagram.
This embodiment provides a discharge lamp having a low color temperature that permits excellent color discrimination (identification) so that it is substantially comparable to a fluorescent lamp, which is used indoors and provides easy color recognition.
Next, a second discharge lamp of the present invention includes a luminous tube in which xenon gas is sealed, and has a light color that lies in a chromatic region common to the following regions: a region bounded by an ellipse with a color point (u, v)=(0.221, 0.329) as a center thereof, a major axis of 0.047, a minor axis of 0.014, and an angle from a u axis of 35 degrees in the CIE 1960 UCS diagram; a region bounded by an ellipse with a color point (u, v)=(0.224, 0.334) as a center thereof, a major axis of 0.040, a minor axis of 0.015, and an angle from a u axis of 15 degrees in the CIE 1960 UCS diagram; and a region bounded by an ellipse with a color point (u, v)=(0.236, 0.339) as a center thereof, a major axis of 0.037, a minor axis of 0.013, and an angle from a u axis of 30 degrees in the CIE 1960 UCS diagram.
This embodiment provides a discharge lamp having a low color temperature range that permits excellent color discrimination (identification) so that it is substantially comparable to a fluorescent lamp, which is used indoors and provides easy color recognition.
In the first and the second discharge lamps of the present invention, the light color preferably lies in a range bounded by lines connecting four color points (u, v) of (0.221, 0.345), (0.252, 0.345), (0.248, 0.338), and (0.231, 0.330) in the CIE 1960 UCS diagram. This embodiment provides a discharge lamp whose light permits a white object such as white lines on roads to be perceived as xe2x80x9cwhitexe2x80x9d when it is illuminated, i.e., a discharge lamp having an excellent property to provide perception of white color.
In the first and the second discharge lamps of the present invention, the light color of the discharge lamp preferably lies in a range bounded by lines connecting three color points (u, v) of (0.224, 0.341), (0.244, 0.341), and (0.229, 0.333) in the CIE 1960 UCS diagram. This embodiment provides a discharge lamp whose light permits a white object such as white lines on roads to be perceived as particularly xe2x80x9cwhitexe2x80x9d when it is illuminated, i.e., a discharge lamp having a particularly excellent property to provide perception of white color.
In the first and the second discharge lamps of the present invention, the color point of the light color preferably lies in a range on the side of color temperature lower than the isotemperature line of a correlated color temperature of 3800K in the CIE 1960 UCS diagram. This embodiment provides another advantage in that the illumination by the discharge lamp is less likely to be dazzling, in addition to the above-described advantages.
In another embodiment of the present invention, the color point of the light color lies in a range on the side of color temperature higher than the isotemperature line of a correlated color temperature of 3400K in the CIE 1960 UCS diagram. This embodiment provides further advantages in that the illumination by the discharge lamp is less likely to be dazzling, and that the color can be discriminated against the light color of a headlight employing a current halogen lamp, in addition to the above-described advantages.
In another embodiment of the present invention, any one of the discharge lamps as described above is used for an automobile headlight. Thus, the automobile headlight can be provided with the advantages described above.
In still another embodiment of the present invention, in any one of the discharge lamps as described above, preferably a metal halide is sealed in the luminous tube.
The discharge lamp of the present invention preferably comprises at least a pair of electrodes. The distance d between the ends of the electrodes is preferably 8 mm or less, the inner diameter D of the luminous tube at the midpoint between the electrodes is preferably 5 mm or less, the power W supplied for illumination is preferably 70 W or less, and the lamp efficiency during illumination is preferably 501 m/W or more.
In another embodiment of the discharge amp of the present invention, the metal halide comprises a halide of sodium, and an amount of the halide of sodium sealed is preferably 50 wt % or more with respect to the total amount of the metal halide sealed.
In still another embodiment of the discharge lamp of the present invention, the metal halide comprises a halide of sodium and a halide of scandium, and does not comprise a halide of thorium nor a halide of thallium, and the following relationship is preferably satisfied:
90 less than WNa+WSc, and
0.75xe2x89xa6WNah/(WSc+WNa) less than 1,
where WNa and WSc represent the percentages by weight of the halide of sodium sealed and the halide of scandium sealed, respectively, with respect to the total amount of the metal halide sealed.
In yet another embodiment of the discharge lamp of the present invention, the metal halide comprises a halide of sodium and a halide of thorium, and does not comprise a halide of thallium, and the following relationship is preferably satisfied:
90 less than WNa+WSc+WTh, and
0.75xe2x89xa6WNa/(WSc+WNa)xe2x88x92WTh/(WSc+WNa+WTh) less than 1,
where WNa, WSc and WTh represent the percentages by weight of the halide of sodium sealed, the halide of scandium sealed, and the halide of thorium sealed, respectively, with respect to the total amount of the metal halide sealed.
In another embodiment of the discharge lamp of the present invention, the metal halide comprises a halide of sodium, a halide of scandium and a halide of thallium, and does not comprise a halide of thorium, and the following relationship is preferably satisfied:
90 less than WNa+WSc+WTi,
0.75 less than WNa/(WSc+WNa) less than 1, and
WTl/(WSc+WNa+WTl)xe2x89xa60.03,
where WNa, WSc and WTl represent the percentages by weight of the halide of sodium sealed, the halide of scandium sealed, and the halide of thallium sealed, respectively, with respect to the total amount of the metal halide sealed.
In still another embodiment of the discharge lamp of the present invention, the metal halide comprises a halide of sodium, a halide of scandium, a halide of thorium, and a halide of thallium, and the following relationship is preferably satisfied:
90 less than WNa+WSc+WTh+WTl,
0.75 less than WNa/(WSc+WNa)xe2x88x92WTh/(WSc+WNa+WTh) less than 1, and
xe2x88x920.05xe2x89xa6WTh/(WSc+WNa+WTh)xe2x88x922xc3x97WTl(WSc+WNa+WTh+WTl),
where WNa, WSc, WTh and WTl represent the percentages by weight of the halide of sodium sealed, the halide of scandium sealed, the halide of thorium sealed and the halide of thallium sealed, respectively, with respect to the total amount of the metal halide sealed.
In yet another embodiment of the discharge lamp of the present invention, the metal halide preferably comprises 10 wt % or less of a halide of cesium with respect to the total amount of the metal halide sealed.
In another embodiment of the discharge lamp of the present invention, the discharge lamp preferably comprises an outer tube having a linear transmittance for light at 350 nm of 30% or less and a linear transmittance for light at 450nm of 70% or more.
In still another embodiment of the discharge lamp of the present invention, the outer tube preferably seals at least one end of the luminous tube.
In yet another embodiment of the discharge lamp of the present invention, an inner diameter of the outer tube is preferably 12 mm or less.
The preferred embodiments of the present invention described above provides a discharge lamp that permits excellent color discrimination, is hardly dazzling, allows an illuminated white object to be perceived as being xe2x80x9cwhitexe2x80x9d, and has a light color that is discriminated against the light color of a current halogen lamp. In addition, the discharge lamp can emit light with a constant color and luminous flux because the temperature at the coldest point of the luminous tube hardly change with illumination conditions.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.